The invention relates to an apparatus making it possible to modify the polarization of an electromagnetic wave and its application to microwave antenna systems and in particular to Cassegrain antennas.
Apparatus making it possible to modify the polarization of an electromagnetic wave are known under the generic term of "polarizers". There are two types of polarizers, the first operating in transmission and the second operating by reflection. The invention relates essentially to the second type of polarizer.
A polarizer by reflection operating at a wavelength .lambda. essentially comprises two facing elements, whereof the radio spacing is approximately .lambda./4, the first element or reflector being constituted by a conducting surface and a second element constituted by a polarization filter.
Reflection polarizers are in particular described in the article by P. W. Hannan "Microwave antennas derived from the Cassegrain telescope" published in I.R.E.Transactions of Antennas and Propagation, March 1961 in which the polarization filter is constituted by a network of small diameter metal wires arranged in parallel in the thickness or on the surface of a dielectric material plate. This type of polarizer has certain disadvantages, particularly the difficulty of mechanically constructing the network of wires with a sufficient accuracy, a higher cost of manufacture and a certain relative fragility. To obviate these disadvantages, it has been proposed, particularly in French Pat. No. 1,499,206 to construct a polarization filter constituted by a network of metal plates physically integral with the reflector.
This polarizer is able to operate in a wider frequency band than the wire polarizer, however, the manufacturing cost remains high and the thickness of the polarizer leads to certain limitations regarding its use.
Polarizers have applications in microwave antenna systems and in particular in antenna systems of the Cassegrain type, which are described in detail in the above-mentioned article by P. W. Hannan.
In Cassegrain antennas, particularly those in which the polarizer is also used for orienting the radiated wave beam, the polarizer must rotate the polarization plane of the microwave by 90.degree., must have a low inertia and limited thickness and must be constructed as to be relatively inexpensive.